Filling-level indicator

ABSTRACT

In a filling-level indicator (12), especially for fuel tanks (10) of motor vehicles, flexural waves of the frequency (f) are fed into a sound conductor (11) via a transmitter (13). For the temperature compensation of the zero point of the filling-level indicator (12), the temperature coefficient of the modulus of elasticity (TKE) of the material of the sound conductor (11) will be at least of the order of magnitude of the negative amount of the temperature coefficient of the longitudinal extension (TKL). Furthermore, it has been shown to be especially advantageous to feed the flexural waves at a frequency f opt  ≈50 kHzmm/d, (d) being the thickness of the sound conductor (11). A temperature compensation of the measuring sensitivity of the sound conductor is possible as a result of this measure. The filling-level indicator thereby supplies relatively accurate measurement values and is virtually independent of temperature influences within a wide temperature range.

BACKGROUND OF THE INVENTION

The invention relates to a filling-level indicator for fuel tanks andcomprising a sonic transmitter, a sonic receiver, and a transmittingbody extending into a fuel tank. It is already known to introduce soundinto a sound-conducting solid body and determine the variation in thepropagation velocity (phase velocity) of flexural waves with a receiver.The propagation velocity of the flexural waves in the solid body is afunction of the height of the fluid level. The propagation velocity inthe solid body in an empty fluid vessel serves as a reference value.Additional measurements are necessary to obtain this function.Furthermore, the transmitter and receiver should not come in contactwith the fluid, otherwise disadvantages arise due to deficient sealingof the transmitter or the receiver or inadequate resistance of thepoints of adhesion. Also, the zero point and the measuring sensitivityof the filling-level indicator are temperature-dependent.

SUMMARY OF THE INVENTION

The object of the invention is to provide a filling-level indicator,having a very high accuracy of measurement and in which influence oftemperature on the zero point and on the measuring sensitivity of thefilling-level indicator is largely eliminated. The object of theinvention is achieved by providing a transmitting body made of amaterial the temperature coefficient of modulus of elasticity of which,with an opposite sign, has substantially the same order of magnitude asthe temperature coefficient of its linear extension. Thus, there is nolonger any need for a reference stage or for a computational correctionin an evaluation unit, this involving a high outlay. It is possible todo without a hitherto necessary additional determination of theinfluence of a temperature on the transmission body.

The present invention both as to its construction so to its mode ofoperation, together with additional objects and advantages thereof, willbe best understood from the following detailed description of preferredembodiment with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a longitudinal sectional view of thefilling-level indicator according to the invention, and

FIG. 2 shows a measurement diagram.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, 10 denotes a tank for the fuel of a motor vehicle, into whichprojects a transmitting body in a form of a sound conductor 11, of afilling-level indicator 12 for the quantity of fuel in the tank. Thesound conductor 11 has a transmitter 13 and a receiver 14 at its upperend projecting out of the fuel. The transmitter 13 is designed as atransducer especially for ultrasonic waves. Both the transmitter 13 andthe receiver 14 are arranged above the maximum filling height L of thetank 10, so that neither extends into the fuel. The effective fillingheight of the fuel in the tank 10 is designated by h. The transmitter 13and the receiver 14 are connected to an evaluation device 15 not shownin detail.

The transmitter 13 excites in the sound conductor 11 sound waves 17,so-called flexural waves, that is to say transverse waves, thepropagation velocity of which is frequency-dependent. Flexural waves aresolid-borne sound waves on plates or bars, the oscillating particles ofwhich are moved essentially perpendicularly to the plate plane and inthe propagation direction. Furthermore, their propagation velocity inthe region of the filling height h of the fuel is substantially lowerthan in that region L-h of the sound conductor 11 surrounded by the airabove the fuel. As shown in FIG. 1, the sound waves 17 come in contactat the transitional surfaces 18 with the particular medium surroundingthese. The propagation velocity of the sound waves 17 is determined bythe size of the transitional surface 18 and is dependent on thegeometrical form of the sound conductor 11. Depending on thefilling-level height h, there is a greater or lesser variation in thepropagation velocity (phase velocity) of the flexural waves. Thismeasurement effect is also utilized in the filling-level indicatormentioned in the state of the art.

Temperature-dependent sources of error of the zero point of thefilling-level indicator 12 include the temperature dependencies of themodulus of elasticity E of the sound conductor 11, its length L, itsthickness D and the density δ of the material used. It has been shown tobe especially advantageous that the temperature dependence of the zeropoint is compensated when the following is true of the materialproperties of the sound conductor:

    TKE=-TKL,

TKE being the temperature coefficient of the modulus of elasticity E andTKL being the temperature coefficient of the linear extension L of thesound conductor 11. This condition is satisfied very effectively, forexample, by the material Thermelast® 4002 (Vacuumschmelze, Hanau). Thetemperature coefficient of the modulus of elasticity and that of thelongitudinal extension L are given in the following table for variousmaterials:

    ______________________________________                                        Material        TKE [1/K]  TKL [1/K]                                          ______________________________________                                        Copper          -3.2 × 10.sup.-4                                                                   1.7 × 10.sup.-5                              Aluminium       -3.1 × 10.sup.-4                                                                   2.4 × 10.sup.-5                              Steel (unalloyed)                                                                             -2.0 × 10.sup.-4                                                                   1.1 × 10.sup.-5                              Thermelast ® 4002                                                                         -5.0 × 10.sup.-6                                                                   8.5 × 10.sup.-6                              ______________________________________                                    

Furthermore, the influence on the propagation velocity of the flexuralwaves in the sound conductor 11 of the surrounding medium is dependenton the physical characteristics density and compression modulus, of themedium. The propagation velocity of the flexural waves is therefore alsodependent on the sound velocity in the fluid. All these variables aretemperature-dependent, and consequently the influence on theflexural-wave velocity is also temperature-dependent. It has been found,according to the invention, that this measuring error depends closely onthe measuring frequency of the sound waves supplied by the transmitter13. This temperature influence is as much the greater, the nearer thefrequency is to the critical frequency at which the velocity of theflexural wave in the sound conductor surrounded by fluid is equal tothat of the density wave in the fluid.

If the filling-level indicator is used for the tank of a motor vehicle,in general there is a working range of -20° C. to 60° C. For thisworking range, there can be indicated a signal frequency f of thetransmitter 13 at which the mean deviation in relation to the measuringsensitivity given at an assumed mean working temperature of 20° C.becomes the smallest possible. The concept of measuring sensitivity isintended to mean the change in velocity of the flexural waves as aresult of the fluid load. If the frequency f of the transmitter 13 isselected as a function of the fluid data, the material data and thedimensions of the sound conductor, the measuring sensitivity of thefluid-level indicator is then falsified by the above-describedparameters scarcely at all. The data of the fluid and the data of thematerial of the sound conductor are all already fixed. It has nowemerged advantageously that the frequency f_(opt) is approximatelyinversely proportional to the thickness d of the sound conductor 11. Theoptimum signal frequency f_(opt) can be determined from the equationf_(opt) =50 kHz mm/d. This indicated equation applies especially whenthe material used for the sound conductor 11 is Thermelast® 4002 or evensteel. In FIG. 2, the dependence of the measuring sensitivity as afunction of temperature is represented for the three measuringfrequencies f₁ =20 kHz, f₂ =31 kHz and f₃ =40 kHz for this material anda size of 1000*10*1.5 mm³. Petrol is used as a medium here. The changeΔc/Δc_(o) in the flexural-wave velocity c as a result of the load of themedium, is plotted on the y-axis as the relative dependence of themeasuring sensitivity. As is evident from FIG. 2, for the frequency f₂=31 kHz the maximum error in the temperature range of -20° C. to 60° C.is below 0.8%, and therefore a mean error less than ±0.4% is achieved byan evaluation circuit.

If the two above-described methods for the temperature compensation ofthe measuring sensitivity and for the temperature compensation of thezero point are combined, a filling-level indicator independent oftemperature over a wide temperature range is obtained.

While the invention has been illustrated and described as embodied in aspecific embodiment of a filling-level indicator, it is not intended tobe limited to the details shown, since various modifications andstructural changes may be made without departing in any way from thespirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:
 1. A filling-level indicator for afuel tank of a motor vehicle, comprising a transmitter for emittingsound waves; a receiver for receiving the sound waves; and at least onetransmitting body projecting into the fuel tank, which is filled with amedium which level is to be determined, for transmitting the sound wavesfrom said transmitter to said receiver, said transmitting body beingformed of at least one sound-conducting material having a temperaturecoefficient of a modulus of elasticity thereof, with an opposite sign,substantially of the same order of magnitude as a temperaturecoefficient of a linear extension of said at least one transmittingbody. PG,11
 2. A filling-level indicator as set forth in claim 1,wherein said transmitter emits sound flexural waves having a frequencydetermined substantially from an equation

    f.sub.opt =50 kHzmm/d,

wherein d is a thickness of said transmitting body.
 3. A method ofdetermining a filling level in a fuel tank of a motor vehicle, saidmethod comprising the steps of providing a transmitter for emittingsound waves; providing a receiver for receiving the sound waves; andproviding at least one transmitting body for projecting into the fueltank, which is filled with a medium which level is to be determined, fortransmitting the sound waves from said transmitter to said receiver, andformed of at least one sound-conducting material having a temperaturecoefficient of a modulus of elasticity thereof, with an opposite sign,substantially of the same order of magnitude as a temperaturecoefficient of a linear extension of said at least one transmittingbody.
 4. A method as set forth in claim 3, wherein the step of providinga transmitter includes providing a transmitter that emits sound flexuralwaves having a frequency determined substantially from an equation

    f.sub.opt =50 kHz mm/d,

wherein d is a thickness of said transmitting body.